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AbstractIncubation temperature influences a suite of traits in avian offspring. However, the mechanisms underlying expression of these phenotypes are unknown. Given the importance of thyroid hormones in orchestrating developmental processes, we hypothesized that they may act as an upstream mechanism mediating the effects of temperature on hatchling phenotypic traits such as reduced growth and thermoregulation. We found that plasma T3, but not T4 concentrations, differed among newly-hatched wood ducks (Aix sponsa) from different embryonic incubation temperatures. T4 at hatching correlated with time spent hatching, and T3 correlated with hatchling body condition, tarsus length, time spent hatching, and incubation period. In addition, the T3:T4 ratio differed among incubation temperatures up at hatch. Our findings are consistent with the hypothesis that incubation temperature modulates plasma thyroid hormones which in turn influences multiple aspects of duckling phenotype.